The present invention relates to implantable pacemakers capable of pacing and sensing in at least one chamber of the heart. More particularly, the present invention, relates to a programmable dual chamber pacemaker wherein the basic configuration of the pacemaker, e.g., unipolar or bipolar, can be changed, including the grounding configuration and ground potentials used within the pacemaker.
Generally, a heart stimulator, commonly known as a "pacemaker" or "pacer," uses one or two flexible leads having one end connected to the pacer and the other end connected to electrodes placed in close proximity to the heart. These leads are used to stimulate or pace the heart. Also, these leads are used to sense the heart activity by picking up electrical signals from the heart.
In order to properly pace or sense, the pacer has to be able to deliver a stimulating pulse to the heart or sense an electrical signal from the heart, and this requires that there be an electrical return path. If, within a given heart chamber, a unipolar lead is used--containing a single conductor--the return path is the conductive body tissue and fluids. The return path is connected to the pacer by connecting the pacer electrical common or ground to the pacer metal enclosure, typically referred to as the pacer "case." The case, in turn, makes contact with the body tissue and/or fluids.
An alternative solution to using a unipolar lead in a given heart chamber is to use a double lead/electrode in the heart chamber, known as a bipolar lead. In a bipolar lead, a second conductor is spiraled over and insulated from a first conductor along the length of the lead. At the distal end of the lead, one of the conductors is connected to a first electrode, referred to as the "tip" electrode, and the second conductor is connected to a second electrode, referred to as a "ring" electrode. The ring electrode is generally situated 10 to 20 mm from the tip electrode. The tip electrode is typically placed in contact with heart tissue, while the ring electrode is in electrical contact with the blood. Because both body tissue and fluids are conductive, the ring electrode of a bipolar lead, in contact with the body fluids, serves as an electrical return for both pacing and sensing.
As indicated, pacing or sensing using the pacer case or enclosure as part of the electrical return path is known as unipolar pacing or sensing. Pacing or sensing using the lead ring electrode and associated lead conductor as the electrical return path is known as bipolar pacing or sensing.
There are numerous factors to consider when deciding whether unipolar or bipolar pacing and/or sensing should be used. Bipolar pacing has, in general, the advantage of requiring less energy than unipolar pacing. Further, bipolar sensing is less prone to crosstalk and myopotential sensing than is unipolar sensing. (Crosstalk, for purposes of this application, refers to a pacer mistakenly sensing a heart activity in one heart chamber immediately after the other chamber is paced.) Bipolar sensing reduces crosstalk resulting from a pacing stimulus in the opposite chamber. Bipolar pacing is preferred if pectoral stimulation occurs with uniploar pacing or if a pocket infection occurs around the unipolar case electrode.
Unipolar pacing and sensing offers the advantage, in general, of simpler circuitry within the pacemaker and a smaller diameter lead. Some doctors prefer unipolar over bipolar pacing and/or sensing as a function of other implantation and heart conditions. Depending on the lead orientation, unipolar sensing may be better than bipolar sensing. Furthermore, unipolar stimulation may be preferred if diaphragmatic stimulation occurs with bipolar pacing. Usually, the pacer has a unipolar factory-set configuration, but in the last five years some programmable configuration pacers have appeared.
In addition to the conventional unipolar and bipolar sensing configurations, a new sensing configuration has the potential of reducing even more the likelihood of crosstalk. This new configuration utilizes unipolar pacing in both channels, and senses between the ring electrode and the case. See U.S. Pat. No. 4,686,988, entitled "Pacemaker System and Method for Measuring and Monitoring Cardiac Activity and for Determining and Maintaining Capture", by Jason Sholder. Unipolar sensing from ring-to-case has all the advantages of unipolar sensing from tip-to-case, with the added benefit of reduced crosstalk due to its separation distance from the stimulation site. Not only is the crosstalk smaller with this new configuration, but one can readily determine capture just immediately after pacing (capture is defined as the heart contracting as a result of a pacer-delivered stimulus.)
As the number of configuration options and their combinations increases, especially with respect to dual chamber pacers (those designed to pace and/or sense in both chambers of the heart), it is clear that pacing and sensing programmability is very important. However, because a pacer is a low voltage, low power consumption device, the implementation of the switching circuitry needed to realize the different pacing and sensing configurations is very difficult. To illustrate, in order to have a very low power consumption device, pacers use integrated circuits with CMOS digital circuits and MOS analog switches and amplifiers. Further, low voltage, power and polarity requirements dictate the use of a P-well CMOS process. (A pacer is typically a positive ground system inasmuch as negative pacing pulses must be generated.) A difficulty with this CMOS process, and the resulting CMOS currents, is that no input, output or any internal transistor drain or source can go above V.sub.DD or below V.sub.SS, where V.sub.DD is the positive supply voltage and V.sub.SS is the negative supply voltage. (For a single battery configuration, V.sub.DD is thus usually obtained from the positive battery terminal, and V.sub.SS from the negative battery terminal.) Because the battery of a pacemaker is typically a single 2.8 volt (V) lithium cell, whose voltage may decrease over its life to as low as 2.0V, this limitation makes it extremely difficult to design pacemaker circuits that will work properly in all output (pacing) and sensing configurations.
In a typical design, the pacer electrical common, or ground reference, is connected to the positive terminal of the battery. In turn, this ground reference is connected to the CMOS IC substrate. The negative terminal of the battery, which for a typical design is -2.8V, thus provides the V.sub.SS supply voltage for the pacer circuits. As pacing magnitudes greater than 2.8V are often required, a voltage adjusting circuit is used in conjunction with a storage capacitor for each channel of the pacemaker in order to produce these higher magnitude voltages. Also, such voltage adjusting circuits, or equivalent, can be used to produce some other higher magnitude voltages needed for circuits which have node voltages of greater magnitude than -2.8V.
However, even though voltage adjusting circuits can be used to produce needed voltages of greater magnitude than is available from the battery, a major problem still exists for nodes having voltages going above V.sub.DD or ground. An example will illustrate how such voltages occur. A pacer delivers a stimulating current pulse by switchably connecting the electrode tip, through a coupling capacitor, to the negative terminal of a storage capacitor, the positive terminal of this capacitor being grounded. The voltage stored on this storage capacitor has previously been adjusted or amplified to the desired magnitude by a voltage adjusting circuit. A coupling capacitor is required to prevent DC current from flowing through the tip electrode body interface. The return path for the pacing pulse is provided by grounding the case or ring electrode for unipolar or bipolar pacing, respectively. After delivering the pulse, the coupling capacitor remains charged with a positive charge on its tip electrode side (distal side). The pacer side of the coupling capacitor (proximal side) would likewise have a charge remaining thereon, but this charge is removed by connecting it through a discharging resistor (or switch) to ground. If, after pacing, it is desired to sense bipolarly between tip and ring, switching means must be used to connect the two inputs of a differential amplifier to the tip and ring electrodes. However, the tip potential remains above ground and the ring potential, situated in close proximity to the tip, has a potential somewhere between ground potential and the tip potential, but definitely above ground. As mentioned, no solid state switch of the type employed in pacer circuits (e.g., CMOS switch) can go above ground. Hence, a problem exists of how to switchably connect the positive (above ground) potentials of the tip and ring electrodes to the sense amplifier.
One possible solution to this problem is to connect the sensing amplifier to the proximal (negative) side of the coupling capacitor, which proximal side will have a potential below ground due to the discharge current through the discharging resistor or switch. This approach has the drawback, however, of applying the capacitor's discharging voltage slope to the sensing amplifier. Further, the ring electrode would have to be connected through an additional coupling capacitor in order to eliminate its voltage potential above V.sub.DD (ground). Requiring an additional discrete component, such as a capacitor, is very undesirable.
A further possible solution to this switching problem would be to have the system ground different from V.sub.DD, e.g., midway between V.sub.DD and V.sub.SS. However, doing so would require a midway voltage source, to produce the midway ground potential, that is buffered by a low output impedance buffer to sustain the high current demands of pacing. Alternatively, the system ground could be connected to -2.8V, the negative battery potential. However, doing so would require at least one more stage to the voltage adjusting circuit in order to produce the negative voltages required for pacing. The addition of such additional circuitry is undesirable because it would increase the bulk and power consumption of the pacer, as well as decrease its reliability.